1. Field of the Invention
This invention resides in the field of steel alloys, particularly those of high strength, toughness, corrosion resistance, and cold formability, and also in the technology of the processing of steel alloys to form microstructures that provide the steel with particular physical and chemical properties.
2. Description of the Prior Art
Steel alloys of high strength and toughness and cold formability whose microstructures are composites of martensite and austenite phases are disclosed in the following United States patents, each of which is incorporated herein by reference in its entirety:
U.S. Pat. No. 4,170,497 (Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an application filed Aug. 24, 1977
U.S. Pat. No. 4,170,499 (Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an application filed Sep. 14, 1978 as a continuation-in-part of the above application filed on Aug. 24, 1977
U.S. Pat. No. 4,619,714 (Gareth Thomas, Jae-Hwan Ahn, and Nack-Joon Kim), issued Oct. 28, 1986 on an application filed Nov. 29, 1984, as a continuation-in-part of an application filed on Aug. 6, 1984
U.S. Pat. No. 4,671,827 (Gareth Thomas, Nack J. Kim, and Ramamoorthy Ramesh), issued Jun. 9, 1987 on an application filed on Oct. 11, 1985
U.S. Pat. No. 6,273,968 B1 (Gareth Thomas), issued Aug. 14, 2001 on an application filed on Mar. 28, 2000
The microstructure plays a key role in establishing the properties of a particular steel alloy, and thus strength and toughness of the alloy depend not only on the selection and amounts of the alloying elements, but also on the crystalline phases present and their arrangement. Alloys intended for use in certain environments require higher strength and toughness, and in general a combination of properties that are often in conflict, since certain alloying elements that contribute to one property may detract from another.
The alloys disclosed in the patents listed above are carbon steel alloys that have microstructures consisting of laths of martensite alternating with thin films of austenite, and the alloys disclosed in U.S. Pat. No. 4,619,714 are low-carbon dual-phase steel alloys. In some of the alloys disclosed in these patents, the martensite is dispersed with fine grains of carbides produced by autotempering. The arrangement in which laths of one phase are separated by thin films of the other is referred to as a xe2x80x9cdislocated lathxe2x80x9d structure, and is formed by first heating the alloy into the austenite range, then cooling the alloy below a phase transition temperature into a range in which austenite transforms to martensite, accompanied by rolling or forging to achieve the desired shape of the product and to refine the alternating lath and thin film arrangement. This microstructure is preferable to the alternative of a twinned martensite structure, since the lath structure has greater toughness. The patents also disclose that excess carbon in the lath regions precipitates during the cooling process to form cementite (iron carbide, Fe3C) by a phenomenon known as xe2x80x9cautotempering.xe2x80x9d The ""968 patent discloses that autotempering can be avoided by limiting the choice of the alloying elements such that the martensite start temperature Ms, which is the temperature at which the martensite phase first begins to form, is 350xc2x0 C. or greater. In certain alloys, the autotempered carbides add to the toughness of the steel while in others the carbides limit the toughness.
The dislocated lath structure produces a high-strength steel that is both tough and ductile, qualities that are needed for resistance to crack propagation and for sufficient formability to permit the successful fabrication of engineering components from the steel. Controlling the martensite phase to achieve a dislocated lath structure rather than a twinned structure is one of the most effective means of achieving the necessary levels of strength and toughness, while the thin films of retained austenite contribute the qualities of ductility and formability. Obtaining such a dislocated lath microstructure rather than the less desirable twinned structure is achieved by a careful selection of the alloy composition, which in turn affects the value of Ms.
In certain applications, steel alloys are needed that maintain strength, ductility, toughness, and corrosion resistance over a very broad range of conditions, including very low temperatures. These and other matters in regard to the production of steel of high strength and toughness that is also resistant to corrosion are addressed by the present invention.
It has now been discovered that carbon steel alloys with a triple-phase crystal structure offer high performance and corrosion resistance over a broad range of conditions. The triple-phase crystal structure is a unique combination of ferrite, austenite, and martensite crystal phases in which crystals of ferrite are fused with crystals that contain the dislocated lath structure disclosed in the prior art patents referenced above, i.e., laths of martensite alternating with thin films of austenite. This triple-phase structure can be formed in various ways, extending over a wide range of compositions and formed by a variety of processing routes that include different types of casting, heat treatment, and rolling or forging. The alloy composition used in creating the triple-phase structure is one which has a martensite start temperature of about 300xc2x0 C. or above, and preferably about 350xc2x0 C. and above. This will ensure that a dislocated lath martensite structure will be included as part of the overall microstructure. To help achieve this, the carbon content is a maximum of 0.35% by weight.
The preferred method for forming the microstructure involves the metallurgical processing of a single carbon steel alloy composition by a process of staged cooling from an austenite phase. The first cooling stage of this method consists of a partial recrystallization of the austenite phase to precipitate ferrite crystals and thereby form a dual-phase crystal structure of austenite and ferrite crystals. The temperature reached in this first cooling stage determines the ratio of austenite to ferrite, as readily seen by the phase diagram of the particular alloy. Once this temperature is achieved, the steel is subjected to hot working to achieve further homogenization and reduction, as well as forming or shaping as desired, depending on the desired final product. Hot working may be performed by controlled rolling, such as for example for ultimate products that are rounds or flats, or by forging to produce distinct shapes, such as blades, agricultural implements, helmets, heli-seats, and the like. After hot working at this intermediate temperature, the second stage cooling occurs, in which the austenite phase is converted to the dislocated lath structure by converting the majority of the austenite to martensite while retaining a portion of the austenite as thin films that alternate with the laths of martensite. This second cooling stage is performed rapidly to prevent the formation of bainite and pearlite phases and interphase precipitates in general (i.e., precipitates along the boundaries separating adjacent phases). Minimum cooling rates in this regard may vary with differences in the alloy composition, but are readily discernible in general from transformation-temperature-time phase diagrams that exist for each alloy. An example of such a diagram is presented herein as FIG. 3 and discussed below.
The resulting triple-phase crystal structure provides a steel alloy that has superior properties over conventional steels in terms of stress-strain relationships, impact energy-temperature relationships, corrosion performance, and fatigue fracture toughness. These and other objects, features, and advantages of the invention will be better understood by the description that follows.